<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
  <script>
      /*
         1.提示用户输入出生的年月。
         2.显示用户今年多少岁了。
      */
      var year = prompt('请输入您的出生年月');
      var age = 2021 -year;
      alert("您今年" + age +"岁了");
  </script>  
</body>
</html>